Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Statement of Comprehensive Income [Abstract]
Unrealized Gain (Loss) on Derivatives	$ 0.2	$ 0.0	$ 0.2	$ 0.0
Net income	36.7	37.9	66.0	79.5
Other comprehensive income, before tax:
Change in net unrealized gains during period	34.9	20.2	74.1	53.8
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity	0.0	0.0	0.0	(6.0)
Change in net actuarial loss	0.2	0.2	0.4	0.3
Other comprehensive income, before tax	34.9	20.0	73.9	47.5
Deferred tax expense related to other comprehensive income	(9.5)	(5.1)	(19.7)	(12.4)
Other comprehensive income, net of tax	25.4	14.9	54.2	35.1
Comprehensive income, net of tax	$ 62.1	$ 52.8	$ 120.2	$ 114.6